A photoreceptor for electrophotography has a structure in which a photosensitive layer having a photoconductive function on an electrically conductive substrate, as a fundamental structure. In recent years, research and development has been actively carried out on organic photoreceptors for electrophotography which use organic compounds as functional components responsible for the generation or transportation of charges, in view of advantages such as the diversity of materials, high productivity and safety, and application of the organic photoreceptors to copying machines, printers and the like is underway.
In general, photoreceptors are required to have a function of retaining surface charges in the dark, a function of receiving light and generating charges, and a function of transporting the generated charges, and photoreceptors are classified into so-called single layer type photoreceptors which have a single layer of photosensitive layer combining these functions; and so-called laminated type photoreceptors which include functionally separated layers such as a charge generation layer that is mainly in charge of a function of charge generation at the time of light reception, a charge transport layer that is in charge of a function of retaining surface charges in the dark and a function of transporting the charges generated at the charge generation layer at the time of light reception, and a photosensitive layer.
The photosensitive layer is generally formed by applying, on an electrically conductive substrate, a coating liquid prepared by dissolving or dispersing a charge generating material, a charge transport material and a resin binder in an organic solvent. In these organic photoreceptors for electrophotography, particularly in the layer that serves as the outermost surface, polycarbonate is often used as the resin binder because polycarbonate is strongly resistant to the friction that occurs between the layer and paper or a blade for toner removal, has excellent flexibility, and has good permeability of exposure light. Among others, bisphenol Z type polycarbonate is widely used as the resin binder. Technologies of using such a polycarbonate as a resin binder are described in Patent Document 1 and the like.
On the other hand, the mainstream of recent electrophotographic apparatuses is constituted of so-called digital instruments which use monochromatic light of argon, helium-neon, a semiconductor laser, a light emitting diode or the like as an exposure light source, and which are capable of digitalizing information such as images and characters to convert the information into light signals, irradiating an electrically charged photoreceptor with light to form an electrostatic latent image on the surface of the photoreceptor, and visualizing the latent image using toner.
Methods for electrically charging a photoreceptor include non-contact charging systems such as a scorotron, in which a charging member and a photoreceptor are not brought into contact; and contact charging systems using a roller or a brush, in which a charging member and a photoreceptor are brought into contact. Among these, the contact charging systems are characterized in that since corona discharge occurs in the proximity of the photoreceptor, less ozone is generated and the applied voltage may be lower as compared with the non-contact charging systems. Accordingly, the contact charging systems are more compact and are capable of realizing electrophotographic apparatuses at lower cost while causing less environmental contamination, and therefore, the contact charging systems constitute the mainstream particularly in medium-size and small-size apparatuses.
As the means for cleaning the photoreceptor surface, scraping off using a blade, a simultaneous development and cleaning process, and the like are mainly used. Cleaning using a blade involves scraping off untransferred residual toner on the surface of an organic photoreceptor using the blade, and may collect the toner into a waste toner box or return the toner into the development machine. Cleaners of such a scraping-off system using a blade require a collection box for recovered toner or a space for recycling, and the full-up of the toner collection box should be monitored. Furthermore, when paper dust or external additives remain on the blade, scratches may occur on the surface of the organic photoreceptor, causing shortening of the service life of the electrophotographic photoreceptor. Thus, there are occasions in which the toner is collected during the development process, or a process for magnetically or electrically suctioning any residual toner adhering to the surface of the electrophotographic photoreceptor is provided immediately before the development roller.
Furthermore, in the case of using a cleaning blade, it is necessary to enhance the rubber hardness or to increase the contact pressure in order to increase the cleaning properties. Therefore, abrasion of the photoreceptor is accelerated so that a fluctuation in the potential and a fluctuation in the sensitivity occur, image aberrance is caused, and flaws occur in the balance of color or reproducibility in color machines.
On the other hand, in the case of using the cleanerless mechanism by which simultaneous development and cleaning is carried out in a development apparatus using the contact charging mechanism, there occurs toner with a fluctuating amount of charging at a contact charging mechanism unit. Meanwhile, in the case where there is toner with reverse polarity which has been incorporated in a very small amount, there is a problem that this toner cannot be sufficiently removed from the surface of the photoreceptor and contaminates the charging apparatus.
Furthermore, the photoreceptor surface is also contaminated by ozone, nitrogen oxides and the like that are generated at the time of photoreceptor charging. There are problems such as image bleeding due to the contaminants themselves, a decrease in lubricity of the surface caused by adhering materials, easy adhesion of paper dust and toner, squealing of the blade, peeling, and the susceptibility of the surface to scratches.
Furthermore, in order to increase the toner transfer efficiency in the transfer process, attempts have been made to reduce residual toner through an increase in the transfer efficiency, by regulating the transfer current to be optimal in accordance with the temperature and humidity environment or the characteristics of paper. Furthermore, as an organic photoreceptor appropriate for such processes or contact charging systems, an organic photoreceptor having improved toner releasability, or an organic photoreceptor that is less affected by transfer, is required.
In order to solve these problems, methods for ameliorating the outermost surface layers of photoreceptors have been suggested. For example, Patent Document 2 and 3 suggest methods of adding filler to the surface layer of a photosensitive layer in order to enhance the durability of the photoreceptor surface. However, in a method of dispersing a filler in such a film, it is difficult to uniformly disperse the filler. Furthermore, as there occurs generation of filler aggregates, a decrease in the permeability of the film, or scattering of the exposure light by the filler, charge transport or charge generation is carried out non-uniformly, and image characteristics are deteriorated. Furthermore, methods of adding a dispersing material in order to enhance filler dispersibility may be mentioned, but since the dispersing material itself affects the characteristics of the photoreceptor, it is difficult to obtain a good balance between durability and filler dispersibility.
Furthermore, Patent Document 4 suggests a method of incorporating a fluororesin such as PTFE into the photosensitive layer. Patent Document 5 suggests a method of adding a silicone resin such as an alkyl-modified polysiloxane. However, in the method described in Patent Document 4, a fluororesin such as PTFE has low solubility in solvents and has poor compatibility with other resins, so that the fluororesin undergoes phase separation and causes light scattering at the resin surface. For that reason, the photosensitive layer does not satisfy the sensitivity characteristics required of a photoreceptor. Furthermore, the method described in Patent Document 5 has a problem that because the silicone resin bleeds into the coating surface, the effects cannot be obtained continually.
Thus, in order to solve such problems, Patent Document 6 suggests a method of enhancing wear resistance by using a resin having a siloxane structure added to the terminal structure. Furthermore, Patent Document 7 suggests a photoreceptor containing a polycarbonate or a polyallylate, both of which have been produced using a phenol compound having a specific siloxane structure, as a starting material. Patent Document 8 suggests a photoreceptor containing a resin in which a siloxane resin structure containing a carboxyl group has been introduced into the resin structure. Also, Patent Document 9 suggests a photosensitive layer containing a polycarbonate which has a silicone structure and has decrease surface energy. Patent Document 10 suggests a photoreceptor containing a polyester resin which includes a polysiloxane as a constituent unit, at the outermost surface layer of the photoreceptor.
Patent Document 11 suggests using a polyallylate as a resin binder of the photosensitive layer, and extensive investigations have been carried out for the purpose of an enhancement of durability or mechanical strength. Patent Document 12 suggests a photoreceptor which uses a phenol-modified polysiloxane resin as a siloxane component, and uses a polycarbonate or polyallylate resin having a siloxane structure in the photosensitive layer. Furthermore, Patent Document 13 suggests an electrophotographic apparatus which includes a photosensitive layer containing a silicone-modified polyallylate resin.
On the other hand, for the purposes of protecting the photosensitive layer, enhancing the mechanical strength, enhancing the surface lubricity, and the like, there have been suggested methods of forming a surface protective layer on the photosensitive layer. However, in these methods of forming a surface protective layer, there are problems that it is difficult to form a film as a charge transport layer, or it is difficult to achieve a sufficiently good balance between the charge transport performance and the charge retention function.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 61-62040    Patent Document 2: JP-A No. 1-205171    Patent Document 3: JP-A No. 7-333881    Patent Document 4: JP-A No. 4-368953    Patent Document 5: JP-A No. 2002-162759    Patent Document 6: JP-A No. 2002-128883    Patent Document 7: JP-A No. 2007-199659    Patent Document 8: JP-A No. 2002-333730    Patent Document 9: JP-A No. 5-113670    Patent Document 10: JP-A No. 8-234468    Patent Document 11: JP-A No. 2005-115091    Patent Document 12: JP-A No. 2002-214807    Patent Document 13: JP-A No. 2004-93865